One of the most effective and efficient methods of deploying high-speed digital services to business and residential customers is to use one of the many forms of DSL (Digital Subscriber Loop) technologies over copper telephone wires. This approach has become very popular in the last 20 years due to the fact that copper wires are already deployed almost everywhere and are quite easy to access, both at the Central Office (CO) and at the Remote Terminal (RT) or the Customer Premises Equipment (CPE).
However, one of the main limitations of DSL technology is that the data capacity of copper wires decreases significantly as the length of the copper loop increases. Therefore, service providers may not be able to offer services with high data speeds over copper wires to customers located more than a few kilometers from the Central Office.
One way to mitigate this issue is to use multiple copper pairs to the same customer location, thereby increasing the total data rate of the resulting multi-pair copper link. This method is typically referred to as “bonding” of copper pairs.
Another significant limitation of DSL technology is the significant spectral interference between DSL services deployed on different twisted copper pairs in the same cable. Spectral interference between different high-bit rate services in a copper cable is caused by the fact that each copper pair acts as an antenna. The signal transmitted on each copper pair, which is intended for the receiver located at the other end of that copper pair, is also inadvertently picked up by all of the neighboring copper pairs, because those pairs are not individually shielded from each other. This creates the well-known phenomenon of “crosstalk”, aptly named for the effect it caused in the early days of the telephone, when the telephone discussion taking place on one line could sometimes be overheard by the people conversing on a different line.
Due to the physical characteristics of copper pairs, and in particular due to the average length of the twist between the two copper wires making up each pair, the crosstalk coupling between different pairs increases exponentially with the frequency of the transmitted signal. But this crosstalk coupling is only one of the three factors that determine the strength of crosstalk. The other two are the strength of the disturbing transmitter (typically referred to as a “disturber”) and the sensitivity of the disturbed receiver (typically referred to as a “victim”) at any given frequency. For example, if the transmit frequency band of the disturber is different than the receive frequency band of the victim, then there will be almost no crosstalk.
Crosstalk typically consists of Near-End Crosstalk (NEXT), caused by disturbers located at the “near-end”, i.e., on the same side (network side or customer side) of the copper loop as the victim's receiver, and Far-End Crosstalk (FEXT), caused by disturbers located at the “far-end”, i.e., on the opposite side of the copper loop from the victim's receiver.
The problem of crosstalk is often more pronounced in systems utilizing bonding of copper pairs, because a bonded link always involves multiple copper pairs, and these multiple copper pairs have the potential of generating crosstalk to each other.
The severe deterioration in the data capacity of copper wires due to crosstalk has sparked significant innovation in crosstalk-reduction methods. These methods typically aim to reduce self-crosstalk and/or alien crosstalk, are defined as follows:
“Self-crosstalk”, which consists of self-NEXT and self-FEXT, is generated by transmitters connected to other lines that are physically connected to the same DSL equipment as the victim's line on at least one side of the loop (i.e., on the CO side or the RT side).
“Alien crosstalk”, which consists of alien NEXT and alien FEXT, is generated by transmitters connected to lines that are physically connected to different DSL equipment than the victim's line on both sides of the loop (i.e., on both the CO side and the RT side).
In the case of self-crosstalk, the fact the disturbing lines and disturbed lines are connected to the same DSL equipment implies that the signals transmitted on the disturbing lines are known. This means that the disturbed lines can take advantage of that knowledge to essentially cancel self-crosstalk. This basic principle has been implemented in various forms of self-crosstalk cancellation, such as the three methods discussed below.
Gigabit Ethernet links transmit data at a rate of 1 Gbps in each direction (upstream and downstream) by bonding four copper pairs, with 250 Mbps of data transmitted on each of the four pairs, with the upstream and downstream transmitters using the same frequency band. The strong self-NEXT generated between the four copper pairs is cancelled on each side of the loop using time-domain filters that essentially form a 4×4 matrix echo canceller.
SHDSL (Single-pair High-Speed DSL) bonded links use multiple copper pairs, each of which may carry data at a different symmetric bit rate, with the upstream and downstream transmitters using the same frequency band. The self-NEXT generated between these copper pairs may be cancelled on each side of the loop using time-domain filters that essentially form an N×M matrix echo canceller, where N is the number of copper pairs in the bonded link, and M is the number of crosstalk sources that are cancelled. Using the same approach, self-NEXT can also be cancelled between lines that do not belong to the same bonded link, at least in the case where the disturbing transmitters and victim receivers are connected to the same DSL equipment on that side of the loop.
VDSL (Very-high-speed DSL) uses different frequency bands for upstream and downstream transmission. Therefore, lines that carry VDSL services do not generate self-NEXT to each other. However, since VDSL is typically deployed on short loops, self-FEXT is a significant concern. Self-FEXT can be eliminated between lines belonging to the same bonded link by building a matrix receiver that utilizes the crosstalk from other modems as part of the main received signal. Self-FEXT can also be cancelled between lines that do not belong to the same bonded link, even if the disturbing transmitters and victim receivers are only connected to the same DSL equipment on one side of the loop. This means, for example, that self-FEXT can be cancelled between DSL lines serving different customer locations, as long as they originate from the same DSL equipment on the CO-side of the loop. Self-FEXT cancellation is possible because the disturbing signals are available in the same location, either on the transmitter side for downstream signals, or on the receiver side for upstream signals. Therefore, downstream self-FEXT can be cancelled by pre-coding the transmitted signals with crosstalk-cancelling additional signals, and upstream self-FEXT can be cancelled by decoding the received signals and subtracting the crosstalk effects of each of those received signals from the other received signals.
In the case of alien crosstalk, the modems on the disturbed lines do not have access to the signals transmitted on the disturbing lines. Therefore, alien crosstalk cannot be precisely cancelled. However, its effects can be mitigated. This may be achieved by correlating the received crosstalk noise across multiple receivers, and removing the correlated part of the noise from at least some of these receivers, thereby increasing the data capacity of the corresponding lines. This type of correlation-based scheme may result in noticeable performance benefits as long as the number of significant crosstalk sources (i.e., the number of strongly disturbing alien transmitters) is lower than the number of disturbed receivers whose noises are correlated.
All of these crosstalk cancellation and crosstalk mitigation schemes result in increased Signal-to-Noise Ratio (SNR) for the corresponding victim receivers. This SNR increase is typically used to increase the bit rate of the victim receiver. Alternatively, the signal power of the corresponding transmitter may be reduced using Power Back-Off (PBO) methods; this results in reduced spectral interference of the corresponding line to neighboring receivers while maintaining the original bit rate. These two approaches may also be combined, converting part of the increased SNR to increased bit rate while also reducing the transmitted power to reduce spectral interference.
However, the benefits of crosstalk reduction are still subject to the inescapable tradeoff between performance and robustness. A receiver that is operating with increased bit rate and/or reduced transmit signal power as a result of self-crosstalk cancellation or alien crosstalk mitigation may be impacted more by the addition of new noise sources on neighboring lines, especially if those new noise sources cannot be cancelled or mitigated. For example, a DSL line benefitting from crosstalk reduction may be impacted more by new DSL services added in a public telephone network than a DSL line that does not use crosstalk reduction.
The reason for this increased sensitivity to new noise sources can be traced back to the nonlinear relationship between noise power and data capacity. When a DSL line is operating in a crosstalk-free environment, its SNR and therefore its data capacity is typically limited only by the signal attenuation of the corresponding copper pair and by the internal noise floor of its own receiver. As new DSL lines are added to the same operating environment, their crosstalk noise raises the noise floor of the original line. What is important, however, is that the first few noise sources typically result in a significant reduction in the data capacity of the original line, while noise sources added later on typically have a much less adverse effect. This is because the data capacity of a DSL line or any other communication channel is a logarithmic function of the ratio of received signal power and received noise power. When the environment is free of crosstalk and other interference noise, the total noise power on a DSL receiver is typically very small, on the order of 1 nW (one nanoWatt). Every new DSL line that is added on a neighboring copper pair generates crosstalk noise with total power on the order of 100 nW. Therefore, the first crosstalk source may decrease the SNR by about 20 dB, while the second crosstalk source may only decrease the SNR by an additional 3 dB. In typical DSL deployment scenarios, the first three to four disturbers added in the operating environment may reduce the SNR by about 25 dB or more, while the next three to four disturbers may only decrease the SNR by an additional 3-4 dB.
When crosstalk reduction methods are utilized in DSL networks, some of the DSL lines in those networks may operate in a nearly crosstalk-free environment. Therefore, when new crosstalk sources (i.e., new DSL lines) are added to their operating environment, these DSL lines are subject to significant SNR reductions as discussed above. If the SNR increase resulting from the crosstalk reduction had been used to either increase the bit rate or reduce the transmit power or both, the sudden SNR reduction may necessitate a re-training of the affected DSL line, resulting in a temporary interruption of service on that line.
Therefore, it would be highly desirable to find a method that improves the tradeoff between increased performance and reduced robustness on DSL lines that utilize crosstalk reduction methods.